1. Field of the Invention
The present invention relates generally to an apparatus and method for detecting a non-transmission period in a discontinuous transmission mode of a data channel used in a Code Division Multiple Access (CDMA) mobile communication system. Specifically, the present invention relates to an apparatus for calculating a signal-to-interference ratio (SIR) using traffic symbol energy, pilot symbol energy and noise energy measured for a predetermined time period, and using a decoder to calculate a symbol error rate (SER) for use in accurately detecting a non-transmission period.
2. Description of the Related Art
Code Division Multiple Access (CDMA) mobile communication systems have evolved from an Interim Standard-95 (IS-95) specification for voice transmission, into an IS-2000 specification which includes high-speed data transmission as well as voice transmission. In the IS-2000 specification, the term “discontinuous transmission mode (DTX mode)” refers to a mode where data is transmitted by the frame only when transmission data is present. Such a discontinuous transmission mode contributes to a reduction in transmission power and signal noise strength, thereby increasing the entire capacity of the system.
However, in the case where a transmitter transmits data frames in a discontinuous transmission mode, if a mobile terminal fails to detect a non-transmission period, forward power control used in IS-2000 systems to increase the system capacity is not correctly performed, thus missing an advantage of the discontinuous transmission. In the forward power control used in IS-2000, a mobile terminal measures a signal-to-interference ratio (SIR) of a signal transmitted by a base station, and compares the measured SIR with a threshold value every 1.25 ms. As a result of the comparison, if the measured SIR is lower than or equal to the threshold value, the mobile terminal transmits to the base station a power control command for instructing the base station to increase transmission power. If the measured SIR is higher than the threshold value, the mobile station transmits to the base station a power control command for instructing the base station to decrease transmission power. Such a method of maintaining a proper received SIR by comparing the received SIR with a threshold value every 1.25 ms and transmitting a corresponding power control command is known by those skilled in the art as “inner loop power control”.
In addition to the inner loop power control, the mobile terminal uses a method of analyzing the quality of a received data frame, and increasing a threshold value if the received data frame has a poor quality, or decreasing the threshold value if the received data frame has a good quality, to maintain a proper SIR. Such a method of maintaining a proper received SIR by controlling a threshold value is known by those skilled in the art as “outer loop power control”.
In a discontinuous transmission mode, if a mobile terminal fails to detect a non-transmission period of data, the mobile terminal will mistake a non-transmitted frame for a received frame having a low quality, and thus increase a threshold value to be compared with a received SIR. Subsequently, in a data transmission period, the mobile terminal compares the received SIR with the increased threshold value, and transmits a corresponding power control command. As a result, the mobile terminal requests the base station to further increase transmission power. A signal detection method may be applied in such cases, however, it is not possible to apply a general signal detection method to a system with a low SIR, such as the CDMA mobile communication system. The general signal detection method, a technique for measuring energy of a signal and comparing the measured energy with a threshold value, is available only where a channel condition is rarely changed and only Gaussian noise is present, or where a received SIR is high. However, in a mobile communication environment, where changes in the channel condition are significant due to fading and received SIR values which are not high, it is not possible to use general signal detection methods that do not consider changes in channel conditions.
In order to cope with such a problem, the existing IS-2000 uses a power control symbol, which exists even in a non-transmission period, as a reference signal to detect a non-transmission period of a dedicated control channel (DCCH). That is, in the discontinuous transmission mode, the IS-2000 DCCH transmits, even during a non-transmission period, a power control symbol having the same power level as seen during a transmission period, for power control of a mobile terminal. Therefore, by calculating a ratio of the energy level of a power control symbol to the energy level of the other symbols within one frame, it is possible to easily detect a transmission period and a non-transmission period. In order to describe the existing method for detecting a discontinuous transmission period of DCCH, a structure of a typical apparatus for detecting a non-transmission period of DCCH is illustrated in FIG. 1.
FIG. 1 is a block diagram that illustrates an example of a known apparatus for detecting a non-transmission period of DCCH in a conventional CDMA mobile communication system. Referring to FIG. 1, a radio frequency (RF) unit 100 down-converts a received RF signal into the baseband signal, and a pseudo-random noise (PN) despreader 101 despreads the baseband signal with a PN code. An orthogonal despreader 102 despreads the PN-despread signal with an orthogonal code and generates a traffic symbol. A channel estimator 103 estimates channel distortion, consisting mainly of phase distortion, generated while the PN-despread signal passes through a radio channel environment. A mixer 104 then compensates the traffic channel phase using the estimation value.
Therefore, if a searcher 105 of the apparatus in FIG. 1 searches a position having the strongest energy, a first energy measurer 106 measures energy by accumulating energy of traffic symbols excluding power control symbols, by using the phase-compensated signal at the searched position on a frame-by-frame basis. A second energy measurer 107 measures energy of the power control symbol by using the phase-compensated signal at the searched position on a frame-by-frame basis. The measured energies are provided to an energy ratio calculator 108. The energy ratio calculator 108 calculates a ratio of the measured energies, and a comparator 109 compares the calculated energy ratio with a predetermined threshold value, and provides the result to a controller 110. Based on the comparison result, the controller 110 determines whether the DCCH is executing a frame transmission or not, i.e., whether a current period is a transmission period or non-transmission period of data.
However, unlike the DCCH, a supplemental channel (SCH) for use in transmitting high-speed data does not use a power control symbol that can be evaluated in a non-transmission period when the discontinuous transmission mode is applied. Therefore, the SCH cannot use the above-stated method for detecting a non-transmission period of the DCCH.
Accordingly, a need exists for an apparatus and method for detecting a non-transmission period in a discontinuous transmission mode of a data channel used in a Code Division Multiple Access (CDMA) mobile communication system.